For many years now, telecommunications carriers have been deploying packet-switched networks in place of or overlaid upon circuit-switched networks for reasons of efficiency and economy. Packet-switched networks such as Internet Protocol (IP) or Ethernet networks are intrinsically connectionless in nature and as a result suffer from Quality of Service (QoS) problems. Customers value services which are guaranteed in terms of bandwidth and QoS.
Carriers may use Multi-Protocol Label Switching (MPLS) over a layer 2 network to create connection-oriented label switched paths (or tunnels) across the intrinsically connectionless network, and thereby to provide guaranteed QoS and bandwidth services to customers. However, MPLS is a relatively unstable and complex standard and carriers ideally desire an alternative.
It is desired to use Ethernet switches in carriers' networks. Use of Ethernet switches in carriers' networks would have the advantages of interoperability (mappings between Ethernet and other frame/packet/cell data structures such as IP, Frame Relay and ATM are well known) and economy (Ethernet switches are relatively inexpensive compared to IP routers, for example). It would also provide a distinct advantage of being the principal technology used by enterprises that require a wide area network service from a carrier and therefore able to work in a native mode.
However, the behaviour of conventional switched Ethernet networks is incompatible with carriers' requirements for providing guaranteed services to customers. Carriers need networks to be meshed for load balancing and resiliency—ie there must be multiple paths across it—and the ability to perform traffic engineering—ie the ability of the network operator to control the provision of explicitly routed variable bandwidth connections (or tunnels) through which traffic may be directed. This provides operators significant flexibility in that the physical network build is not obliged to correspond to the offered load and therefore is tolerant of changing usage patterns without requiring on going physical modifications.
In contrast, conventional Ethernet networks must be simply-connected—ie there must be one and only one logical path choice between each and every node of the network. As a consequence, conventional Ethernet networks do not have support for network-wide load balancing, suffer from resiliency problems and cannot support traffic engineering. Further the impact of a single failure with respect to the overall load carried can be significant.
Spanning tree protocols are known which enable a physically meshed Ethernet network to be logically transformed into a simply-connected network by detecting physical loops and logically disabling connections to break up the loops. Spanning tree protocols are also known which are able to detect failure of a physical connection (thereby partitioning the fully-connected network) and automatically restore one or more previously-disabled physical connections so as to re-connect the network. This provides a degree of resiliency. However, carriers need to plan their network traffic routes to achieve much higher resiliency, flexibility and efficiency than spanning tree can achieve. This level of routing capability is best achieved by segregating the traffic into connections whose routes are determined as part of this planning process.
Virtual Bridged LANs (or VLANs) are described in the Institute of Electrical and Electronics Engineers (IEEE) standard 802.1Q, 2003 Edition. FIG. 1 shows a conventional VLAN 10 split up into a plurality of component LANs 12 and connected via VLAN-aware Media Access Control (MAC) bridges 14. Component LANs 12 are typically provided for different communities of interest, such as users sharing a common server or having common network protocol requirements. Unique identifiers (VLAN tags or VLAN IDs) are used to identify each component LAN. Broadcast traffic is broadcast only within component LANs. This helps to overcome the scalability issues of Ethernet by partitioning the whole network 10 resources into smaller broadcast domains. VLAN tags are used to distinguish between traffic for different component LANs when forwarding traffic on shared links between MAC bridges. However the size of the standard VLAN tag is limited to 12 bits, which in turn limits the scale of the network and the number of partitions of component LANs to 4094, where two VLAN tags are reserved with special meaning not for general assignment.
The Internet Engineering Task Force (IETF) has published an Internet Draft referred to as draft-kawakami-mpls-lsp-vlan-00.txt. This document describes the use of VLAN tags for label switching across Ethernet networks in a manner similar to use of MPLS labels for label switching over MPLS networks—VLAN tags are used as labels to mark traffic at an ingress point of a label switched path (LSP) as belonging to a Layer 2 tunnel, and VLAN-aware Ethernet switches in the network act as a VLAN label switched routers.
Connections are formed using one or more LSPs. Intermediate nodes along the connection may optionally swap the inbound label to a different outbound label. In this manner the VLAN tag has meaning specific to any given local node, and the ability to reuse VLAN tags solves some of the scalability issues of 802.1Q.
However, one problem with the method proposed in draft-kawakami-mpls-lsp-vlan-00.txt is that only a maximum of 4094 unique VLAN tags are definable in 802.1Q compliant equipment. This still limits the flexibility and increases the complexity of provisioning connections across the network. Another problem is that connections may not easily be re-routed once provisioned without in general creating transitory loops.
Another problem is that since the Frame Check Sequence (FCS) in Ethernet frames is computed over both the payload and header portions of the frame, every time a VLAN tag (ie a label) is swapped at the ingress or egress point of a LSP, the FCS needs to be recomputed since the VLAN tag will have changed. This requires performing a computation function over the entire Ethernet frame. Moreover, during the interval from when the original FCS is removed and the new FCS added, the frame is vulnerable to corruption without the protection of any FCS.
Yet another problem with the ‘label-swapping’ approach proposed in draft-kawakami-mpls-lsp-vlan-00.txt is that it requires a “chain of correctness”, in that forwarding relies on each local label-forwarded link on the LSP being correct. This should be contrasted with conventional Ethernet which uses globally unique address information to perform forwarding As the LSP labels are not globally unique per conventional Ethernet, it is possible for a forwarding fault in performing label translation to be concealed if a value is incorrectly mapped to another value that is in use. More importantly, from a practical perspective, ‘label-swapping’ behaviour represents a significant change from conventional Ethernet switch functionality, and current telecommunications standards.